Structural characterisation of the olive pomace pectic polysaccharide arabinan side chains

An arabinan (97% of Ara and 3% of hexuronic acid) was isolated from the alcohol-insoluble residue (AIR) of olive pomace by treatment with 0.02 M HNO(3), at 80 degrees C, followed by graded precipitation with ethanol. It was separated from acidic pectic polysaccharides by anion-exchange chromatography, and by size-exclusion chromatography its molecular weight was estimated as 8.4 kDa. By methylation analysis, the linkage composition was established as 5:4:3:1 for (1-->5)-Araf, T-Araf, (1-->3,5)-Araf and (1-->3)-Araf, respectively. 13C NMR spectroscopy confirmed this linkage composition, and allowed to assign the alpha anomeric configuration for the arabinofuranosyl residues, except for some terminally linked ones, that were seen to occur as T-beta-Araf. By 2D NMR spectroscopy (1H and 13C), it was possible to conclude that the T-beta-Araf was (1-->5)-linked to a (1-->5)-Araf residue. Also, in the arabinan (1-->5)-Araf backbone, the branched (1-->3,5)-Araf residues were always adjacent to linear (1-->5)-Araf residues. A tentative structure is proposed.